Describe what is meant by "selective permeability."

Selective permeability is a feature of some barriers, in particular the cell membrane (or plasma membrane). It means that some chemical species are allowed to pass the barrier, while others are not.
The selective permeability of the cell membrane allows the membrane to keep needed chemical species inside the cell and exclude potentially harmful species. The membrane owes this ability to its being a lipid bilayer. It is composed of two layers of a particular kind of large molecule, a phospholipid. A phospholipid has a charged phosphate group, which readily interacts with water, while the rest of the molecule is made up of two hydrocarbon chains or "tails," which are called "hydrophobic," because they tend to avoid water. The plasma membrane is a double layer of these phospholipids, lined up with their hydrophobic tails together and their charged ends in contact with the aqueous (water) solutions on either side of the membrane. The membrane is thus like a sandwich, with charged areas on the outside and a hydrophobic area in the middle.
Ions, which have charges, are attracted to water and will not pass through the hydrophobic part of the membrane. Molecules of the size of glucose and larger also cannot pass through the membrane. Water, oxygen, and carbon dioxide are all small, uncharged molecules, and they can pass freely across the membrane and into and out of the cell. Cells do have the ability to transfer glucose and ions into or out of the cell, but these species cannot diffuse freely across the membrane.
Selective permeability is often taught in connection with osmosis. Osmosis is a condition where there is a net movement in one direction across a selectively permeable membrane. Diffusion is random, and a substance that can diffuse across a membrane will cross in both directions equally, if conditions are the same on both sides. Most often we are concerned with water, and we demonstrate the phenomenon with glucose or salt dissolved in the water. If we place distilled water on one side of a selectively permeable barrier, and a solution on the other side, the water will diffuse more slowly from the solution. The rate of transfer from the distilled water to the solution is then greater than the rate of transfer from the solution to the distilled water. This difference in rates means there is a net transfer of water from the distilled water side to the solution, causing the solution to become more dilute.
The greater the concentration of salt, glucose, or other dissolved material, the slower will be the diffusion rate from the solution. If solutions of differing concentrations are placed on the two sides of a selectively permeable barrier, the net flow of water will always be into the more concentrated solution. This can be seen in the laboratory as cells placed in a hypertonic (concentrated) solution will shrink because of the net flow of water outward, while cells placed in a hypotonic (less concentrated) solution will swell.
Fresh water is in short supply in many parts of the world. Selectively permeable membranes make it possible to use reverse osmosis to remove salt from seawater.This is called desalination. It would be more accurate to say they remove the water from the salty seawater. Applying pressure to a salt solution can speed up diffusion of water across a selectively permeable membrane. With enough pressure, the water diffuses faster from the solution than into it, so there is a net flow of water from the seawater to pure water. As the salt cannot cross the membrane, it is left behind, and the salt-free water can be further purified for drinking.